(1) Field of the Invention
The present invention relates to a transmission system for a tractor, particularly an agricultural tractor, in which a transmission gearing including propelling line and power takeoff line change speed mechanisms is operatively connected through a tubular outer shaft and an inner shaft mounted in the outer shaft to a double clutch connected to an engine output shaft.
(2) Description of the Prior Art
A known transmission system as described above is disclosed, for example, in a Japanese utility model application laid open under No. 56-12142. In the known transmission system, the inner and outer shafts operatively connected to the double clutch constitute parts of the propelling and power takeoff lines, respectively. The double clutch is operable by a first level pedal depression to break drive transmission to the propelling line and by a further, second level pedal depression to break drive transmission to the power takeoff line. Thus, a change speed operation for changing a traveling speed of the tractor while engaging in an agricultural or other operation is carried out by the first level pedal depression which breaks only the drive transmission to the propelling line and permits the power takeoff line to remain in a driven state.
Since in the above transmission system the outer and inner shafts have considerable lengths, bearing bushings are often provided between the two shafts in order to maintain a coaxial relationship therebetween. However, when only the drive transmission to the inner shaft is broken to effect the change speed operation on the propelling line, the bushings provided between the two shafts act to transmit to the inner shaft a rotational force of the outer shaft of the power takeoff line remaining in the driven state. This results in the inner shaft rotating together with the outer shaft, whereby the propelling line is maintained at too high a rotational rate for a smooth operation to be effected for changing the traveling speed of the tractor.
In order to overcome the above inconvenience, the known transmission system comprises a propelling line change speed gearing including a vane wheel rotatable in lubricating oil stored in a transmission case. According to this construction, when only the drive transmission to the propelling line is broken, the oil applies a stir resistance to the vane wheel to promptly reduce the rotation of the propelling line.
In the case of utilizing such a stir resistance, however, a desired resistance is secured only by means of a large vane wheel. Therefore, the above construction is available only to tractors having a sufficient accommodating space in the transmission case.